Well, it does not need to be that dramatic (developer passing away).
The lifetime of any software product is limited. Businesses are being sold or shut down. Apps are going away because of whatever reason.
My conclusion is: data that is critical to me is sitting locally on my LAN (Synology) with local and off-site backups - at least in the long run. I do not care how nice whatever app is: no proprietary databases, no cloud, no nothing. It is available directly via the file system or it is not for me (again: in the long run that is).
The asterisk is: in the long run. I enjoy proprietary apps with their databases and I use them, but eventually, for archival reasons pictures, documents and other files are sitting in a place I have control of at all times.
I use Apple Photos, but if photos are really nice, they are being exported to my NAS and the real editing is being done in Lightroom (which honors “my” file system so that I could move on to any other app for my photos with not too much hassle).
I am using Ulysses within my Setup subscription, but important Markdown files are exported to my NAS. And to be honest, I have catched myself again and again creating Markdown in BBEdit in combination with Marked 2 - why bothering with Ulysses in the first place when I am sitting in front of my Mac…
Regarding services: every service that has my data has a COPY of my data. They do not have the ONLY copy of my data.
My way might sound a little bit extreme, but that is what has helped me over and over again when apps or services went away…